[release_2.9-1.6] Update matter_sdk submodule to 987f81a9d6

[silabs-matter-ci-bot]

Bumps matter_sdk submodule from 3d85042a5b to 987f81a9d6.

See diff at SiliconLabsSoftware/matter_sdk@3d85042...987f81a

Manual examination steps

1. Use the above matter_sdk diff link to see what changed. Note which files were added or deleted and see if they were dealt with in the previous steps. If not, update components:
   • Changes under ./src may affect slc/component/matter-core-sdk.
   • Platform changes under ./src/platform/silabs and ./examples/platform/silabs affect slc/component/matter-platform.
   • Non-Silabs platform changes can be ignored.
2. Note if GN build flags or their usage changed, update component config defines if needed.
3. Analyze PR CI results.
4. Analyze build failures if any, and update component definitions to add/remove files affected by the SDK update until CI passes.